1. Field of the Invention
This invention relates to a method of migrating a melt of a metal through a solid body of semiconductor material by thermal gradient zone melting (TGZM) and, in particular, to the uniform initiation of migration by enhancing the penetration of melts into the material at the surface where fine liquid wires are being migrated by minimizing the near-surface reduction in the thermal gradient.
2. Description of the Prior Art
W. G. Pfann in U.S. Pat. Nos. 2,813,048 and 2,739,088 describes methods for practicing the movement of melts of metal through particular regions of a solid body of semiconductor material by thermal gradient zone melting. However, molten line and droplet instability resulted in the breakup of the migrating lines and droplets and consequently acceptable semiconductor devices were not always obtainable.
Recently, Thomas R. Anthony and Harvey E. Cline discovered that preferred planar orientations of the surfaces of the body of semiconductor material, migration axis and line orientation axis relationships were also a necessity to migrate liquid metal wires and/or droplets through the solid body (See U.S. Pat. Nos. 3,899,362 and 3,904,442, for example). These improvements in TGZM resulted in commercialization of the process. However, as the width of the lines being migrated becomes smaller, the penetration of fine liquid lines of less than 2 mils in width, and preferably 1 mil in width, and small liquid droplets, less than 6 mils in diameter, from the surface of a wafer or body of semiconductor material has been difficult to achieve repeatedly on a commercial basis by a thermal gradient alone. Although a thermal gradient is strong enough to cause migration of the small liquid zones once they are formed in the bulk of semiconductor material, the thermal gradient force is not powerful enough to overcome the surface tension forces holding fine liquid zones, or wires, on the surface of a body, or wafer. Despite further improvements to the TGZM processing techniques, including alloying the deposited metal to the surface (U.S. Pat. No. 3,897,277) and sintering of the same (U.S. Pat. No. 4,006,040), the problem still persists as one attempts to migrate fine wires on a commercial basis. As a result, TGZM to date has been limited to line dimensions typical of isolation grids in solid state power devices and has not had any commercial impact on integrated-circuit type devices which require much finer doped region geometries.
Further studies of the problem indicate that a thermal gradient near the surface of a body of semiconductor material is considerably reduced from its value in the bulk material. This is attributed to the partial infrared transparency of the material of the body or wafer. This near-surface reduction in thermal gradient near the cold entrance surface of a wafer of semiconductor material causes liquid zone penetration to become more difficult and may even prevent, completely, penetration of liquid zones or melts less than a critical size.
In a similar manner, a reduction in the thermal gradient near the hot exit surface of a wafer of semiconductor material undergoing thermal gradient zone melting processing prevents the liquid zone or migrating melt from exiting from the solid body. It has been observed in such instances that the melt will migrate through the bulk of the solid material of a silicon wafer of 10 mil thickness and then come to rest at a distance of 1/2 to 1 mil from the exit surface. After a time, the melt may, and often does, break through the exit surface but the time at rest causes a widening of the melt in the region of rest.
A means of eliminating such near-surface gradient effects is to heavily dope the wafer of semiconductor material in the affected near-surface regions to a depth of from 1 to 25 microns depending on the solubility of the dopant migrating material. Although this dopant method prevents the fall-off in the thermal gradient near the surfaces of the wafer, it requires extra diffusion steps and an extra lapping step. This remedy is applicable generally when thermal migration is one of the first process steps. In addition, the heavy doping of the near-surface regions of the wafer induces both dislocation formation and stress in the wafer.
An object of this invention is to provide a method for minimizing the reduction in the thermal gradient near the major opposed surfaces of a body of semiconductor material.
Another object of this invention is to provide a thin opaque layer of a suitable material on one or both major surfaces of body of semiconductor material in order to enhance the retaining of a thermal gradient in the near surface region.
Other objects of this invention will, in part, be obvious and will, in part, appear hereinafter.